1. Field of the Invention
The invention relates to a projection system comprising a radiation source unit for generating a radiation beam, a modulation system for modulating the radiation beam in accordance with a picture to be displayed, and a projection lens system for projecting the picture to be displayed on a projection screen, said modulation system comprising at least one color selective mirror for splitting the radiation beam into sub-beams, and at least a first and a second reflective light valve for reflecting and modulating the sub-beams. The invention also relates to a modulation system for use in such a projection system.
2. Art Background
A projection system is used for large format display of a picture, for example a television or video program but, for example also for visually displaying a picture generated by a computer such as text, illustrations or other data. The picture is projected on a screen which is visible to the viewer, which screen may be a rear projection screen which is connected, for example fixedly to the projection system and forms an assembly therewith, or as a front projection screen which is not directly coupled to the projecter.
A projection system as described in the opening paragraph is known from U.S. Pat. No. 4,191,456. The system described in this patent comprises a light source and a polarization-sensitive beam splitter with which a linearly polarized radiation beam is generated. This beam is split up by means of two color-selective mirrors into three sub-beams for the primary colors blue, green and red. Each sub-beam is directed to a relective light valve via a neutral and a polarization-sensitive mirror. The light valve operates as a mirror in which, depending on the picture to be displayed, the direction of polarization of portions of the reflected beam is rotated. The reflected sub-beam again traverses the polarization-sensitive mirror, while the portions whose direction of polarization is rotated are separated from the rest of the sub-beam. The sub-beams with the rotated polarization are combined to a beam by means of a second set of color-selective mirrors, which beam is projected on the screen by means of a projection lens.
The known projection system is not compact. The radiation paths for the radiation beams to the light valves extend parallel to the radiation paths for the reflected beams. Thus, a large part of the system is duplicative. Particularly, the sysiem comprises two sets of color-selective mirrors. Such mirrors are sensitive to the direction of polarization of the incident radiation so that these mirrors cannot be replaced without any light and quality loss by single sets which is arranged in radiation paths of both the ongoing and the reflected beams.
A further drawback is that the known projection system uses relatively expensive polarization-sensitive beam splitters for separating the beams incident on the light valves and the beams reflected by the light valves. Since tbe optimization of the color-selective mirrors requires that the separation of the incident and reflected beams takes place between the light valves and the color-selective mirrors, these polarization-sensitive beam splitters are necessary.